A portable power source apparatus has a closing surface (21) facing an open end of a main case (10) and an exposed surface (22) externally exposed at an opposite end of a cap section (20) from the closing surface; a planar circuit board (38) housed in the cap section with voltage conversion circuitry for converting the voltage of the circular cylindrical batteries CB contained in the main case; and a power supply terminal (50) disposed in the exposed surface of the cap section and capable of outputting power from the circular cylindrical batteries CB contained in the main case. The circuit board is disposed diametrically across a circular cross-section of the cap section, and a pair of ribs (42) is provided approximately perpendicular to the circuit board.